The invention concerns a method of producing unsaturated diolefinic alcohols (particularly geraniol and nerol) by selective hydrogenation of citral.
Citral is an aldehyde containing two olefinic carbon-carbon bonds and is of the following total formula: ##STR1## 3,7 dimethyl 2,6 octadiene-al or citral (C.sub.10 H.sub.16 O).
Selective hydrogenation of the aldehyde function leads to the formation of unsaturated isomeric alcohols such as geraniol and nerol, which both have the following total developed formula: ##STR2## 3,7 dimethyl 2,6 octadiene-1 ol (nerol or geraniol).
These materials are used particularly in the perfume industry.
It is very difficult with this reaction is to obtain high and, if possible, total conversion with the best possible selectivity, i.e. as close as possible to 100%, for diolefinic alcohols. The isomeric diolefinic alcohols, geraniol and nerol, may in turn undergo partial or total hydrogenation of the double olefinic bonds, to give the following two products: firstly ##STR3## 3,7 dimethyl-6 octene-1 ol or citronellol in the case of hydrogenation of the double bond located at .alpha. in the alcohol function secondly ##STR4## 3,7 dimethyl octanol 1 when all the unsaturated carbon-carbon and carbon-oxygen bonds are hydrogenated.
The double olefinic bond conjugated with the aldehyde function may also be hydrogenated to give a compound of the following total formula: ##STR5## 3,7 dimethyl 7-octenal or citronellal which, after hydrogenation of the double olefinic bond, gives 3,7 dimethyl octanal.
The group VIII metals are known as catalysts for converting citral under hydrogen pressure and in liquid phase, that is to say in the presence of a solvent. Thus selective hydrogenation of citral to citronellal has been claimed in the presence of catalysts based on palladium (EP-A-008741 and FR-B-2405232), as has been claimed hydrogenation of citral to 3,7 dimethyl octanal (DE 2832699). Cobalt and nickel chlorides, associated in solution with potassium cyanide and an aliphatic amine, also enable very high yields of citronellal to be obtained from citral under hydrogen pressure (SU958.408).
The conversion of citral to citronellol is described in a number of publications. Thus a 93% yield of citronellol is obtained under hydrogen pressure from citral in the presence of a catalyst containing palladium associated with iridium, osmium, platinum, rhodium or ruthenium and with an aliphatic amine (DE 293250). Nickel, rhodium, platinum or palladium in solid form enable citronellol to be produced from citral with selectivities of over 90% (Kinet. Catal 21(3), 670-5). Nickel deposited on chromium oxide, in an alcoholic medium and in the presence of sodium carbonate, enables 95% yields of citronellol to be obtained from citral (Reaction Kinetics and Catalysis Letters, 16(4), 339-43).
As far as selective hydrogenation of citral to geraniol and nerol in liquid phase (CH.sub.3 OH) is concerned, the obtaining of high selectivities (93-95%) with limited conversion has been claimed (U.S. Pat. No. 4,100,180) in the presence of platinum oxide PtO.sub.2 promoted by FeSO.sub.4 and Zn (CH.sub.3 COO).sub.2. The catalyst may be reused, provided that the iron and zinc compounds are added with each citral charge. Similarly, a platinum based catalyst supported on charcoal will give high selectivities for geraniol and nerol (over 95%) with 92 and 97% citral conversions (U.S. Pat. No. 4073813). Finally, two patents in the name of BASF AG claim the use of catalysts, one containing 5% by weight of Pd, Ir, Os, Rh, Ru or Pt supported on charcoal in the presence of an aliphatic amine (U.S. Pat. No. 4,455,442), and the other containing 5% of ruthenium supported on charcoal promoted by iron, also in the presence of an aliphatic amine (U.S. Pat. No. 4,465,787). The catalysts have high activity and selectivity but require well-defined additions of chemical compounds.